SIZE AND VERTICAL DISTRIBUTIONS OF STRATOSPHERIC RADIOACTIVE AEROSOLS P. J, DREVINSKY and J. PECCI U.S. Air Force Cambridge Research Laboratories, Bedford, Massachusetts ABSTRACT Four balloon flights were made over Minneapolis, Minn. (45°N 93°W), during 1962 and 1963 to collect radioactive aerosols for the determina- tion of size and vertical distributions over the altitude range of 9 to 30 km. The sampler consisted of a two-stage impactor followed by a polystyrene microfiber filter for collecting the smallest particles. This sampler had been designed to collect particles of about 0.15 yu in radius and larger on the first stage and particles between about 0.02 and 0.15 4 in radius on the second stage on the basis of a particle density of 2 g/ ecm*, With the use of four samplers during each flight, four-point profiles were obtained by collecting samples over altitude intervals of 9 to 15, 15 to 21, 21 to 27, and 27 to 30 km. All samples were measured for total beta radioactivity, Sr, and “Ce, For the 1963 samples Zr was also determined. With few exceptions most of the activity is carried by particles of about 0.02 to 0.15 win radius and shows a peak concentration at altitudes between 15 and 21 km. Limited data for total gamma activity, “Min, and °Ba were also obtained. INTRODUCTION The particle size distribution of artificial radioactivity in the stratosphere is important because of its influence in causing variations in atmospheric concentrations of nuclear debris with respect to time and location and because of its role in the calibration of sampling sys158